In the art of making envelopes from folded sheets, typically but not necessarily of paper, it has long been known to produce square or oblong envelopes from rectangular sheets having triangular flaps on their four corners all of which are folded toward the same face of the sheets.
Certain of these conventional methods include forming an envelope from a rectangular sheet having opposite first and second sheet edges and opposite third and fourth sheet edges with a sheet center point equidistant between the opposite edges. Parallel first and second fold lines extend between the first and third sheet edges and the second and fourth sheet edges respectively and parallel third and fourth fold lines extend at right angles to the first and second fold lines between the second and third sheet edges and the first and fourth sheet edges respectively. This defines a fold rectangle within the edges of the sheet having a center point coincident with the sheet center point, and it also defines first through fourth right triangular flaps beyond the respective first through fourth fold lines with each flap having an apex point at its right angle. The triangular flaps are folded toward the same face of the fold rectangle in any sequence but with the first flap folded last to complete closure and with no apex point of any of the flaps extending beyond the fold line of the opposite flap.
U.S. Pat. No. 2,021,620 to Weir is perhaps the closest single reference disclosing the foregoing conventional practice in folding envelopes from rectangular sheets. Weir teaches opposed pairs of flaps extending beyond respective fold lines which define a fold rectangle and he indicates that the center point of the rectangle should be coincident with the center point of the sheet. His triangular flaps are folded toward the same face of the fold rectangle with no apex point of any of the flaps extending beyond the fold line of the opposite flap.
Certain of these same concepts are disclosed in British Patents Nos. 8285 to Fetherstonhaugh and 553,816 to Chapman and in U.S. Pat. Nos. 4,744,509 to Buchler and U.S. Pat. No. 665,796 to Myers.
Nowhere in the prior art, however, has it been recognized that by application of certain mathematical relationships among the size and dimensions of the sheet and the size and orientation of the fold rectangle and triangular flaps, an envelope can be formed with sheet edge sections on two adjoining flaps exactly aligned and partially coincident and with sheet edge sections on the other two adjoining flaps similarly aligned and partially coincident. In certain oblong forms of envelopes this relationship results in an envelope mouth cleanly defined by two straight sides at right angles to one another, one of which sides is defined by two exactly aligned sheet edge sections on adjoining flaps. Nor did the prior art recognize that the same mathematical relationships can result in the final closure flap overlapping the other flaps with an overlap portion of uniform width throughout its length so that conventional gumming on that overlap portion will result in uniform adhesion throughout its area of contact. Moreover, the prior art did not recognize that the same mathematical relationships can determine the minimum and maximum sheet sizes capable of producing an envelope of a given size with a given overlap width on the final closure flap.